Insertion and elimination. Peter H.M. Budzelaar

Transcription

1 Peter H.. Budzelaar

2 Insertion reactions If at a metal centre you have a) a σ-bound group (hydride, alkyl, aryl) b) a ligand containing a π-system (olefin, alkyne, C) the σ-bound group can migrate to the π-system. C 2

3 Insertion in en(c) 5 insertion HTS LU agostic C C adduct η 2 -acyl 3

4 Insertion reactions The σ-bound group migrates to the π-system. But if you only see the result, it looks like the π-system has inserted into the -X bond, hence the name insertion. To emphasize that it is actually (mostly) the X group that moves, we use the term migratory insertion. The reverse of insertion is called elimination. Insertion reduces the electron count, elimination increases it. Neither insertion nor elimination causes a change in oxidation state. 4

5 1,1 insertions In a 1,1-insertion, metal and X group "move" to the same atom of the inserting substrate. The metal-bound substrate atom increases its valence. C, isonitriles (NC) and S 2 often undergo 1,1-insertion. e C e e S 2 S e 5

6 Insertion of C and isonitriles C insertion is hardly exothermic. An additional ligand may be needed to trap the acyl and so drive the reaction to completion. In the absence of added ligands often fast equilibrium. C insertion in -H, -CF 3, -C endothermic. no C polymerization. but isonitriles do polymerize! 6

7 Double C insertion? Deriving a mechanism from a reaction stoichiometry is not always straightforward. The following catalytic reaction was reported a few years ago: 2 2 NH + 2 C + ArI "Pd" 2 NCCAr + 2 NH 2 + I - This looks like it might involve double C insertion. But the actual mechanism is more complicated. 7

8 No double C insertion! 2 NCCAr L 2 Pd(C) n ArX C - n C red elim ox add L 2 Pd CAr CN 2 L 2 Pd X Ar HN 2 - H + nucl attack CAr L 2 Pd C + subst C - X - L 2 Pd X ins CAr C 8

9 Promoting C insertion "Bulky" ligands C requires more space than Lewis acids Coordinate to, stabilize product AlCl 3 AlCl 3 C vs Drawback: usually stoichiometric 9

10 Sometimes it only looks like insertion Nucleophilic attack at coordinated C can lead to the same products as standard insertion: Ir e Ir e C Ir C e Ir Ce ain difference: nucleophilic attack does not require an empty site. 10

11 1,2-insertion of olefins Insertion of an olefin in a metal-alkyl bond produces a new alkyl. Thus, the reaction leads to oligomers or polymers of the olefin. e 11

12 1,2-insertion of olefins Insertion of an olefin in a metal-alkyl bond produces a new alkyl. Thus, the reaction leads to oligomers or polymers of the olefin. Best known polyolefins: polyethene (polythene) polypropene In addition, there are many specialty polyolefins. Polyolefins are among the largest-scale chemical products made. They are chemically inert. Their properties can be tuned by the choice of catalyst and comonomer. 12

13 Why do olefins polymerize? Driving force: conversion of a π-bond into a σ-bond ne C=C bond: 150 kcal/mol Two C-C bonds: 2 85 = 170 kcal/mol Energy release: about 20 kcal per mole of monomer (independent of mechanism!) any polymerization mechanisms adical (ethene, dienes, styrene, acrylates) Cationic (styrene, isobutene) Anionic (styrene, dienes, acrylates) Transition-metal catalyzed (α-olefins, dienes, styrene) Transition-metal catalysis provides the best opportunities for tuning of reactivity and selectivity 13

14 echanism of olefin insertion Standard Cossee mechanism Green-ooney variation (α-agostic assistance): CH 2 P CH 2 P CH 2 P Interaction with an α C-H bond could facilitate tilting of the migrating alkyl group The "fixed" orientation suggested by this picture is probably incorrect H P H 14

15 Insertion in -H bonds Insertion in -H bonds is nearly always fast and reversible. Hydrides catalyze olefin isomerization egiochemistry corresponds to arkovnikov rule (with δ+ -H δ- ) To shift the equilibrium to the insertion product: Electron-withdrawing groups at metal alkyl more electron-donating than H Early transition metals -C stronger (relative to -H) Alkynes instead of olefins more energy gain per monomer, both for -H and -C insertion 15

16 Catalyzed olefin isomerization etals have a preference for primary alkyls. But substituted olefins are more stable! Cp 2 Zr Cp 2 ZrHCl Cl dominant alkyl dominant olefin Cp 2 Zr Cl In isomerization catalysis, the dominant products and the dominant catalytic species often do not correspond to each other. For each separately, concentrations at equilibrium reflect thermodynamic stabilities via the Boltzmann distribution. 16

17 Catalyzed olefin isomerization Cp 2 ZrHCl xs or Cp 2 ZrCl ost stable alkyl or + + ost stable olefin + little 17

18 -H vs -C insertion Insertion in -C bonds is slower than in -H. Barrier usually 5-10 kcal/mol higher Factor in rate! eason: shape of orbitals (s vs. sp 3 ) 18

19 epeated insertion ultiple insertion leads to dimerization, oligomerization or polymerization. k prop H Et Bu k CT H + Key factor: k CT / k prop = κ κ 1: mainly dimerization κ : oligomerization (always mixtures) κ «0.1: polymerization κ 0: "living" polymerization k prop Hx k prop etc k CT k CT H + c H + k prop For non-living polymerization: κ Nn+ 2 = ( n 0) n+ 1 κ + 1 W n+ 2 ( ) 2 ( n + 2) κ = (2κ + 1) n+ ( κ + 1) 1 19

20 Schulz-Flory statistics ole fraction Weight fraction ole fraction Weight fraction ole fraction Weight fraction κ = 0.7 κ = 0.1 κ = 0.02 Key factor: k CT / k prop = κ κ 1: mainly dimerization κ : oligomerization (always mixtures) κ «0.1: polymerization κ 0: "living" polymerization For non-living polymerization: κ Nn+ 2 = ( n 0) n+ 1 κ + 1 W n+ 2 ( ) 2 ( n + 2) κ = (2κ + 1) n+ ( κ + 1) 1

21 Applications of oligomers and polymers Ethene and propene come directly from crude oil "crackers" Primary petrochemical products, basic chemical feedstocks Dimerization rarely desired aking butene costs $$$! ligomers: surfactants, comonomers High added value, but limited market Polymers: plastics, construction materials, foils and films Very large market, bulk products 21

22 Selective synthesis of trimers etc? 1-Hexene and 1-octene are valuable co-monomers. Selective synthesis of 1-hexene from ethene is not possible using the standard insertion/elimination mechanism. There are a few catalysts that selectively trimerize ethene via a different mechanism ("metallacycle" mechanism). edox-active metals (Ti, V, Cr, Ta) required Cr systems are used commercially There are also one or two catalysts that preferentially produce 1-octene. The mechanism has not been firmly established. 22

23 Trimerization via metallacycles = (and others) Ti II + subst II coord II Key issues: Geometrical constraints prevent β-elimination in metallacyclopentane. Formation of 9-membered rings unfavourable. red elim Ligand helps balance (n) and (n+2) oxidation states.? H IV H IV β-elim coord IV ins IV 23

24 C/olefin copolymerization C cheaper than ethene Copolymer more polar than polyethene much higher melting point Chemically less inert P P C C P No double C insertion uphill No double olefin insertion C binds more strongly, inserts more quickly Slow β-elimination from alkyl 5-membered ring hinders elimination C P C P = L 2 Pd, L 2 Ni 24

25 Hydroformylation Used to make long-chain alcohols and acids from 1-alkenes ften in situ reduction of aldehydes to alcohols Unwanted side reaction: hydrogenation of olefin to alkane ain issue: linear vs branched aldehyde formation It is possible to make linear aldehydes from internal olefins! H H C C H 2 H H H 2 H 25

26 Insertion of longer conjugated systems Attack on an η-polyene is always at a terminal carbon. LU coefficients largest Usually α,ω-insertion 26

27 Insertion of longer conjugated systems A diene can be η 2 bound. 1,2-insertion etallocenes often do not have enough space for η 4 coordination: 27

28 Diene rubbers Butadiene could form three different "ideal" polymers: cis 1,4 trans 1,4 1,2 In practice one obtains an imperfect polymer containing all possible insertion modes. Product composition can be tuned by catalyst variation. Polymer either used as such or (often) after cross-linking and hydrogenation. 28

29 Addition to enones Li, Grignards: usually 1,2 "charge-controlled" H rganocu compounds often 1,4 or even 1,6 etc "orbital-controlled" stereoregular addition possible using chiral phosphine ligands frequently used in organic synthesis 29

30 Less common elimination reactions α-elimination: Cp 2 Zr - Cp 2 Zr H tbu H Probably via σ-bond metathesis: Zr H ther ligand metallation reactions: tbu H Zr - Zr L 2 Pt - L 2 Pt Via σ-bond metathesis or oxidative addition/reductive elimination 30

31 Less common elimination reactions β-elimination from alkoxides of late transition metals is easy: CH3 + CH 2 H The hydride often decomposes to H + and reduced metal: alcohols easily reduce late transition metals. Also, the aldehyde could be decarbonylated to yield metal carbonyls. For early transition metals, the insertion is highly exothermic and irreversible. 31